1. Field of the invention
The present invention relates generally to a DPCM predictive encoder. More specifically, the invention relates to a DPCM predictive encoder employing a differential PCM system for predictive encoding of a NTSC television image signal.
2. Description of the Related Art
In the predictive encoder for a NTSC color television image signal, a NTSC color television signal is initially converted into a digital signal by an analog/digital converter (A/D converter). A sampling clock frequency of the A/D converter is set at four times of a color sub-carrier frequency, for example. The digital signal is compared with a prediction signal generated in a predictor by an differential value generator so that an differential signal is generated. The differential signal is quantized by a quantizer for encoding. An encoding output is added to the prediction signal of the above-mentioned predictor by an adder so that a local decoding signal is generated. This local decoding signal becomes an input for the predictor to generate the prediction signal.
As a predictor, one frame period predictor constructed with one frame delay circuit, is employed, which generates a pixel information for a subject pixel to be encoded depending upon the corresponding pixel in one previous frame.
Here, it is known in the art that the one frame period predictor is used for obtaining prediction effect, in which an differential signal in a frame period relative to a still image becomes zero. When such one frame period predictor is used in the DPCM predictive encoder employing the A/D converter operating at four times of the sub-carrier frequency, the interframe differential signal in the still image becomes zero to provide effective prediction result. However, if the sampling frequency of the A/D converter is set at three times of the sub-carrier frequency, a sampling phase relationship between the frames is differentiated so that inteframe predictor cannot be effective even on the still image and the differential signal in one frame period cannot be made zero. Therefore, in this case, the interframe prediction is not effective for the still image.
It is an object of the present invention to provide a DPCM predictive encoder which enables interframe prediction to be effective when the A/D converter is acting at the speed three times of the color sub-carrier.